Contact cements or adhesives are polymer solutions or dispersions which are applied to two surfaces, dried, and the mating surfaces are then pressed together, usually without heat. The dried surfaces are essentially non-tacky and will not adhere to most materials except to another coating of the same adhesive. Care must be used in aligning the two articles or surfaces to be bonded, since the bonding is essentially instantaneous and the articles cannot be moved relative to one another in order to put them into register once contact has been made. This "instant-grab" property of contact adhesives has the important advantage that long periods of aging or oven curing are not needed. For on-site applications, such as laminating a plastic surface sheet material to a kitchen countertop, the advantages of the use of contact adhesives are particularly valuable.
As is shown by U.S. Pat. No. 2,976,203 and 2,976,204 to Young (assigned to the assignee of the present application) it is known to utilize chain transfer agents to lower the molecular weight of acrylic latex polymers useful as pressure sensitive adhesives and as contact cements. Another reference showing that chain transfer agents are known for making pressure sensitive adhesives in U.S. Pat. No. 3,806,484, in which the acrylic monomers are pre-emulsified along with the chain transfer agent, or, as understood, the emulsion is polymerized in increments wherein the chain transfer agent is added in the same quantity to each stage or increment of the emulsion being polymerized.
A patent relating to synthetic rubber, U.S. Pat.No. 4,145,494, shows the polymerization of diolefins, optionally with minor proportions of monoethylenically unsaturated monomers such as acrylic acid, methacrylic acid, itaconic acid, esters thereof, styrene, etc., in emulsion, during which large amounts of chain transfer agents are added after at least 75% of the monomers are polymerized. A shortstopping agent is added to prevent complete conversion of the monomer to polymer. No such agent is used in the present invention.
Contact adhesives differ from pressure sensitive adhesives in that they are essentially non-tacky (although they adhere to one another), while pressure sensitive adhesives retain a permanent, aggressive tack. This may be illustrated by comparing tack ratings of contact and pressure sensitive adhesives in conventional tack tests such as the Rolling Ball Tack Test. In this test, a 7/16" steel ball is rolled down a 6 inch, 45.degree. inclined steel sheet onto the horizontal adhesive surface in the form of a dry coating of about 1 mil in thickness. A pressure sensitive adhesive is expected to stop the ball after it has travelled only one or two inches across the adhesive surface. Conversely, contact adhesive will permit the steel ball to roll almost indefinitely.
Traditionally, contact adhesives have been based on solutions of poly(chloroprene) or neoprene, in combinations of solvents such as toluene and methyl ethyl ketone. Recent government regulations restricting use of such solvents has sparked adhesive manufacturers to seek alternate systems which pose significantly lower flammability and pollution hazards.
Aqueous adhesives seem to be ideal candidate replacements for the hazardous solvent systems. However, early aqueous neoprene latices lacked the high bond strengths shown by their solvent counterparts, and they are not stable to freezing and thawing. In a subsequent innovation, it was found that the deficiency could be overcome by using specialty acrylic latices such as 87.5 ethyl acrylate/10 methyl methacrylate/2.5 itaconic acid with a poly(vinyl alcohol) thickener and a benzoguanamine-formaldehyde condensate crosslinker. While this may be used without safety hazards to provide high strength wood-to-plastic laminates, the system requires higher lamination pressure than customers desire, and thus greater combinability was sought. Improved combinability, as evidenced by good adhesion with low lamination pressure, can be achieved by incorporation of tackifiers and plasticizing solvents, reducing Tg, or dropping molecular weight. However, in all cases combinability is achieved along with an unacceptable loss in elevated temperature performance.
Extensive testing of contact adhesives has indicated that performance in terms of adhesive properties such as lap shear, high temperature cleavage, legginess, combinability, etc., is related to the type and amount of acid functionality in the polymer backbone and the type and amount of crosslinker. However, it is apparent that performance is even more dramatically related to molecular weight of the polymer. High molecular weight improves lap shear adhesion and cleavage properties, but downgrades legginess, combinability, and bond fusion. The exact opposite is true of low molecular weight polymers.